1. Field of the Invention
The present invention relates to a ceramic material and a method for producing therefor; in particular, it relates to a ceramic material for solid-state electrolyte applicable to various secondary batteries such as lithium ion secondary battery and air secondary battery, and to a method for producing therefor.
2. Description of Related Art
In recent years, with the development of portable devices such as personal computers and mobile phones, there has been rapidly increasing demand for secondary batteries usable as a power source thereof. In cells of the secondary batteries used in such applications, a liquid electrolytes (an electrolytic solution) containing a combustible organic diluent solvent are commonly used as an ion transfer medium. In batteries using such electrolytic solution, there is the possibility that problems such as leaking of the electrolyte solution, ignition, explosion and the like may occur.
Thus, from the point of view of ensuring safety intrinsically, advances are being made in the development of all-solid-state secondary batteries, in which solid-state electrolytes are used in place of liquid electrolytes, while at the same time, all battery elements are constituted by solids. Such all-solid-state secondary batteries have the advantages in that, there is no danger of ignition or leakage due to being ceramics sintered with electrolytes, and moreover, problems such as deterioration of the battery capability due to corrosion do no occur readily. Among these, all-solid-state lithium secondary batteries using lithium metals for the electrodes are believed to be secondary batteries that can easily be given a high energy density.
In addition, in order to improve the battery performance of a secondary battery, the keys thereof are the enlargement of the difference in electric potential between the materials used for the positive electrode and the negative electrode, and the improvement of the volume density of each material used in the positive and negative electrodes. In particular for negative electrode materials, it is known that using Li metals or Li alloys has an important contribution to improving the battery performance. However, Li metals in the negative electrode, sometimes causes the phenomenon of precipitation of dendrite which is a whisker like crystalline along with charge-discharge behavior, penetrates the separator and causes a short inside the battery at the worst. Therefore, Li metals could not be used for the negative electrode due to safety issues. In an all-solid-state battery in which the electrolyte portion is formed by a solid-state electrolyte, since deposits cannot penetrate the solid-state electrolyte, it is anticipated to be usable safely. However, since this Li metal has high reactivity and lowest electric potential, so a useable solid-state electrolyte comprising a ceramic material has not been discovered heretofore.
In recent years, it has been reported that Li7La3Zr2O12 (hereinafter referred to as LLZ), a ceramic material of the garnet-type, had excellent lithium resistance, and that there was a possibility of being usable as a solid-state electrolyte for an all-solid-state Li secondary battery (see Ramaswamy Murugan et al., Angew. Chem. Int. Ed. 2007, 46, 1-5).
However, when the present inventors attempted to produce a LLZ pellet based on Ramaswamy Murugan et al., Angew. Chem. Int. Ed. 2007, 46, 1-5, a LLZ pellet usable as a solid-state electrolyte for an all-solid-state lithium secondary battery could not be obtained, and the material did not qualify as being practically usable as a solid-state electrolyte material for an all-solid-state lithium secondary battery.